1. Field of the Invention
The present invention relates to a silicon nitride sintered material and to a production process thereof, and more particularly to a silicon nitride sintered material which exhibits excellent mechanical characteristics and anti-corrosion property, which has a high thermal expansion coefficient, and which has high heat resistance so as to make the sintered material suitable for use as an insulating material such as a base material used in a ceramic glow plugs, as well as to a production process thereof.
2. Description of the Related Art
Silicon nitride sintered material, having excellent mechanical characteristics and heat resistance, has been employed as an insulating material for use in ceramic heaters in which a resistance heater is embedded or for use in similar products. In this case, increase in the weight of silicon nitride sintered material through oxidation is desired as low as possible, so that the sintered material attains excellent anti-corrosion property. When silicon nitride sintered material is used as an insulating material, a problem arises in that cracks are possibly generated in the insulating material during application or generation of heat, because silicon nitride has a thermal expansion coefficient lower than that of tungsten, tungsten carbide, molybdenum silicide, or a similar substance that is generally employed as a resistance heater (i.e. a resistance heating element) embedded in the insulating material. Therefore, in order to prevent generation of cracks, the thermal expansion coefficient of the insulating material must be substantially as large as that of the resistance heater. Thus, when the silicon nitride sintered material is used as an insulating material for ceramic heaters including a glow plug, the sintered material must have both excellent anti-corrosion property and a high thermal expansion coefficient.
In order to increase the thermal expansion coefficient of the insulating material, particles of high thermal expansion coefficient compounds such as rare earth element compounds, metallic carbides, metallic nitrides, metallic silicides, etc., having a thermal expansion coefficient higher than that of the silicon nitride have been conventionally incorporated into a raw material powder of silicon nitride, and have been dispersed therein. Typically, such a high thermal expansion coefficient compound incorporated into the silicon nitride sintered material is in an amount of a few % to about 30% by volume.
However, incorporation of a rare earth element compound that has a higher thermal expansion coefficient than a silicon nitride deteriorates the anti-corrosion property of the sintered material, particularly anti-corrosion property at about 1,000xc2x0 C., because formation of a crystalline phase having oxy-nitride, such as an H (RE20Si12N4O48) phase, a J (RE4Si2N2O7) phase, or an M (RE2Si3N4O3) phase (RE: rare earth element) is formed during firing. Therefore, use of the sintered material as an insulating material for ceramic heaters is problematic. Thus, in order to prevent deterioration of the anti-corrosion property of the sintered material, conventionally, studies have been carried out on the compositions and particle sizes of silicon nitride raw material and sintering aids. However, obtaining silicon nitride sintered material having a high thermal expansion coefficient while maintaining excellent anti-corrosion property has remained difficult so far.
In view of the foregoing, an object of the present invention is to provide a silicon nitride sintered material which exhibits excellent mechanical characteristics and anti-corrosion property, which has a high thermal expansion coefficient and which has high heat resistance so as to make the silicon nitride sintered material suitable for use as an insulating material such as a base material for ceramic glow plugs, as well as a production process thereof.
The present inventors have performed studies on the relation of components of silicon nitride sintered material and amounts thereof vs. insulating property and thermal expansion coefficient thereof; and have found that when the amount of a rare earth element as reduced to a certain oxide thereof, the element being contained in the sintered material, is determined so as to fall within a specific range, and when the ratio by mol of subtraction remainder oxygen amount as calculated in relation to the oxygen contained in the sintered material, the remainder oxygen amount being expressed in terms of silicon dioxide, to the amount of oxygen contained in the sintered material is determined so as to fall within a specific range, the silicon nitride sintered material has a high thermal expansion coefficient, and exhibits excellent anti-corrosion property and mechanical characteristics. The present invention has been accomplished on the basis of this finding.
The present invention provides a silicon nitride sintered material comprising silicon nitride, any of Group 4a through 6a elements, a rare earth element, and silicon carbide, characterized in that the amount of the rare earth element as reduced to a certain oxide thereof is 5.7-10.3 mol %, and the ratio by mol of subtraction remainder oxygen amount as calculated in relation to the oxygen contained in the sintered material, the remainder oxygen amount being expressed in terms of silicon dioxide, to the amount of oxygen contained in the sintered material is at least 0.50 and less than 0.70.
The present invention also provides a silicon nitride sintered material comprising silicon nitride, any of Group 4a through 6a elements, a rare earth element, and silicon carbide, characterized in that the amount of the rare earth element as reduced to a certain oxide thereof is 15-26 mass %, the amount of said any of Group 4a through 6a elements as reduced to a certain oxide thereof is 5-13.5 mass %, and the amount of the silicon carbide is 0.8-3 mass %.
The present invention also provides a silicon nitride sintered material comprising silicon nitride, any of Group 4a through 6a elements, a rare earth element, and silicon carbide, characterized in that the amount of the rare earth element as reduced to a certain oxide thereof is 5.7-10.3 mol %, and a crystalline phase of the sintered material contains no J phase.
The present invention also provides a silicon nitride sintered material comprising silicon nitride, any of Group 4a through 6a elements, a rare earth element, and silicon carbide, characterized in that the ratio by mol of subtraction remainder oxygen amount as calculated in relation to the oxygen contained in the sintered material, the remainder oxygen amount being expressed in terms of silicon dioxide, to the amount of oxygen contained in the sintered material is at least 0.50 and less than 0.70, and a crystalline phase of the sintered material contains no J phase.
The present invention also provides a silicon nitride sintered material produced by firing a raw material powder mixture containing silicon nitride powder, powder of a rare earth element compound, powder of a compound of any of Group 4a through 6a elements, and silicon carbide powder, wherein the amount of the rare earth element as reduced to a certain oxide thereof is 15-26 mass %, and the amount of said any of Group 4a through 6a elements as reduced to a certain oxide thereof is 5-13.5 mass %.
The present invention also provides a process for producing a silicon nitride sintered material, characterized by preparing a raw material powder mixture by mixing silicon nitride powder, powder of a rare earth element compound, powder of a compound of any of Group 4a through 6a elements, and silicon carbide powder, such that the amount of the rare earth element as reduced to a certain oxide thereof is 15-26 mass %, and the amount of said any of Group 4a through 6a elements as reduced to a certain oxide thereof is 5-13.5 mass %; and firing the raw material powder mixture.
In the silicon nitride sintered material of the present invention, examples of the aforementioned xe2x80x9cGroup 4a through 6a elementxe2x80x9d include Ti, Ta, Mo, W, and Cr. Of these, Cr is particularly preferred. No particular limitation is imposed on the amount of the Group 4a through 6a element contained in the sintered material, and the amount of the Group 4a through 6a element is usually 5-13.5 mass %, preferably 6-10 mass %, more preferably 6.5-10 mass %, as reduced to a certain oxide thereof. When the amount of the element falls within the above ranges, the mechanical characteristics at high temperatures can be improved.
Examples of the aforementioned xe2x80x9crare earth elementxe2x80x9d contained in the silicon nitride sintered material of the present invention include Eu, Sm, Y, Sc, La, Ce, Pr, Nd, Gd, Tb, Dy, Er, and Yb. In the sintered material of the present invention, the aforementioned xe2x80x9cthe amount of the rare earth element as reduced to a certain oxide thereofxe2x80x9d is usually 5.7-10.3 mol %, preferably 6-9.5 mol %, more preferably 6-9 mol %. When the amount of the rare earth element as reduced to a certain oxide thereof is less than 5.7 mol %, since the thermal expansion coefficient of the silicon nitride sintered material decreases, cracks are generated in an insulating material as a result of the difference in thermal expansion coefficient between the insulating material and a resistance heater during application of heat or generation of heat, which is not preferable. In contrast, when the amount of the rare earth element as reduced to a certain oxide thereof exceeds 10.3 mol %, the anti-corrosion property and flexural strength of the silicon nitride sintered material are impaired, which, again, is not preferable. As used herein, xe2x80x9cthe amount of a rare earth element as reduced to a certain oxide thereofxe2x80x9d refers to the amount of a rare earth element contained in the sintered material as reduced to RE2O3 (RE: rare earth element). The rare earth element content is 15-26 mass %, preferably 15-25 mass %, more preferably 16-24 mass %, much more preferably 17-24 mass %, as reduced to a certain oxide thereof. When the rare earth element content falls within the above ranges, the mechanical characteristics at high temperatures can be improved.
In the silicon nitride sintered material of the present invention, the aforementioned xe2x80x9csubtraction remainder oxygen amount as calculated in relation to the oxygen contained in the sintered materialxe2x80x9d refers to the amount of oxygen (which amount is obtained by subtracting, from the amount of oxygen contained in the sintered material, the amount of oxygen that is contained in corresponding oxides of rare earth elements contained in the sintered material when the rare earth elements are expressed as the oxide) as expressed in terms of silicon dioxide (SiO2). In the silicon nitride sintered material of the present invention, the ratio by mol of the remainder oxygen amount in relation to the sintered material as reduced to silicon dioxide to the amount of oxygen contained in the sintered material (i.e., SiO2/[SiO2+RE2O3], RE: rare earth element) is at least 0.50 and less than 0.70, preferably 0.50-0.695. When the ratio is less than 0.50, anti-corrosion property is deteriorated, which is not preferable, whereas when the ratio is 0.70 or more, further improvement of anti-corrosion property is difficult, and flexural strength is lowered, resulting in lowering of strength of the sintered material, which is not preferable.
No particular limitation is imposed on the amount of the aforementioned xe2x80x9csilicon carbidexe2x80x9d contained in the silicon nitride sintered material of the present invention, but the amount is usually 0.8-3 mass %, preferably 1-3 mass %, more preferably 1.5-2.5 mass %. When the amount falls within the above ranges, lowering of insulation resistance, which is attributed to conductivity of silicon carbide, is prevented. In addition, aciculation of particles of silicon nitride, which is an insulating substance, is prevented, and therefore the specific surface area of the silicon nitride particles increases, resulting in prevention of formation of paths for conduction of electricity by conductive particles of a high thermal expansion coefficient compound.
The silicon nitride sintered material of the present invention contains a rare earth element, but does not contain a four-component crystalline phase of rare earth element-silicon-oxygen-nitrogen. Examples of the aforementioned xe2x80x9cfour-component crystalline phase of rare earth element-silicon-oxygen-nitrogenxe2x80x9d include an H (RE20Si12N4O48) phase, a J (RE4Si2N2O7) phase, and an M (RE2Si3N4O3) phase (RE: rare earth element) as defined by JCPDS (Joint Committee On Powder Diffraction Standards). Although a crystalline phase such as the H phase, J phase, and M phase, particularly the phase, has a high thermal expansion coefficient, the crystalline phase causes deterioration of anti-corrosion property of the sintered material, particularly anti-corrosion property at about 1,000xc2x0 C., due to the crystalline phase containing oxy-nitrogen that is decomposed into oxygen and nitrogen at such high temperature. Therefore, when the silicon nitride sintered material does not contain xe2x80x9ca four-component crystalline phase of rare earth element-silicon-oxygen-nitrogen,xe2x80x9d the sintered material can maintain excellent anti-corrosion property (i.e. corrosion resistance at high temperature). Other crystalline phases such as mono-silicate phase (RESiO5) not containing oxy-nitrogen do not aggravate the corrosion resistance.
The silicon nitride sintered material of the present invention, having the aforementioned structure and, exhibits excellent mechanical characteristics and anti-corrosion property, and has a high thermal expansion coefficient. Specifically, the thermal expansion coefficient of the silicon nitride sintered material is usually at least 3.7 ppm/xc2x0 C., preferably at least 3.8 ppm/xc2x0 C., more preferably at least 3.85 ppm/xc2x0 C., between room temperature and 1,000xc2x0 C. When the thermal expansion coefficient is less than 3.7 ppm/xc2x0 C., in a case where the sintered material is used as an insulating material such as a base material of ceramic glow plugs, cracks are generated in the insulating material during application or generation of heat, because of poor thermal expansion of the sintered material, and such cracks are attributed to the difference in thermal expansion coefficient between the insulating material and a resistance heater formed from tungsten, tungsten carbide, molybdenum silicide, or similar material. The flexural strength of the sintered material as measured by means of a four-point flexural strength test according to JIS R1601 (1981) is at least 750 MPa, preferably at least 800 MPa, more preferably at least 900 MPa. The anti-corrosion property (increase in the weight through oxidation) of the sintered material as measured by means of a method described in the Embodiments below is 0.04 mg/cm2 or less, preferably 0.3 mg/cm2.
No particular limitation is imposed on the production process for the silicon nitride sintered material of the present invention, but the sintered material can generally be produced as follows:a raw material powder mixture is prepared by mixing silicon nitride powder, powder of a rare earth element compound, powder of a Group 4a through 6a element compound, and silicon carbide powder, such that the amount of the rare earth element as reduced to a certain oxide thereof is 15-26 mass %, and the amount of the Group 4a through 6a element as reduced to a certain oxide thereof is 5-13.5 mass %; and subsequently the raw material powder mixture is fired. No particular limitation is imposed on the aforementioned xe2x80x9crare earth element compoundxe2x80x9d and xe2x80x9cGroup 4a through 6a element compound,xe2x80x9d so long as the former contains a rare earth element and the latter contains a Group 4a through 6a element. Typical examples of xe2x80x9cthe rare earth element compoundxe2x80x9d employed include a certain oxide of a rare earth element (RE2O3, RE: rare earth element). Typical examples of xe2x80x9cthe Group 4a through 6a element compoundxe2x80x9d employed include silicides (CrSi2, Cr5Si3, etc.) and oxides of the element. No particular limitation is imposed on the particle size of the aforementioned xe2x80x9csilicon carbide powderxe2x80x9d to be incorporated, but the average particle size is usually 1 xcexcm or less, preferably 0.7 xcexcm or less, more preferably 0.1-0.7 xcexcm. When the average particle size falls within the above ranges, the specific surface area of the silicon carbide increases, and the silicon carbide greatly exerts the effect of preventing aciculation of silicon nitride particles. The average particle size of the silicon carbide refers to the average particle size of incorporated silicon carbide raw material, provided that grains are not formed from silicon carbide particles through sintering.
No particular limitation is imposed on the firing method and firing conditions for producing the silicon nitride sintered material of the present invention, so long as the sintered material can be produced. Firing may be carried out at ambient pressure or at high pressure. The firing temperature is usually 1,650-1,950xc2x0 C. In order to prevent decomposition of silicon nitride, firing is usually carried out in a non-oxidizing gas atmosphere containing nitrogen.